For such a large city (35 million people in the metropolitan area) Tokyo is surprisingly not as polluted as it might be. Still, it's far from perfect, so it's good to hear that the Tokyo Metropolitan Government is involved in a project to use biodiesel derived from biological sources in its public buses.

Unlike some other alternative fuels, biodiesel - made from sources such as vegetable oil and animal fat - can be used in unmodified diesel engines, making it more likely to gain widespread acceptance. The Tokyo project, which is being run with several partners, including Toyota , aims to replace 10 percent of the fuel in buses with biodiesel.

In the UK a 5 percent biodiesel blend is generally supported by car manufacturers, while the likes of Volkswagen in Europe maintain warranties even on diesel vehicles running on 100 per cent biodiesel. The overall aim of the Japanese plan is to cut carbon emissions to a quarter of their 200 equivalent by 2020.